It is known that the drawing of tubes by extrusion of a metal billet is carried out in an hydraulic press, the ingot or billet of cylindrical shape being placed in a receptacle of the same cross-section provided inside a container; the billet is pushed by a hydraulic jack against a die, within which a coaxial mandrel makes an annular space for formation of the tube. In so-called direct-drawing presses, the container is fixed and the billet is pushed against the die which closes the front face of the receptacle of the container by means of a rammer which penetrates via the rear face of the receptacle and which consists of a rod provided at its end with a thrust insert, the diameter of which is equal to that of the receptacle.
In so-called reverse-drawing presses, the rammer is fixed and consists of a tube provided at its end with an insert having a diameter equal to that of the receptacle of the container and carrying the die. The receptacle of the container is closed at its other end by a shutter, and the assembly is displaced towards the rammer in such a way that the latter slips into the receptacle for the billet. For the drawing of a tube, the shutter is perforated with an orifice through which passes the mandrel which penetrates through the billet and slips into the die.
The drawing press will therefore consist essentially of a fixed part comprising a bed and a fixed cross-member, which are spaced from one another and connected by columns, and of a movable part comprising a movable cross-member which is actuated by a main jack bearing on the bed and on which the container is supported.
Whether drawing is carried out directly or in reverse, a mandrel, the end of which penetrates into the die, must always pass through the billet. It is possible to use pre-perforated billets, i.e., those provided beforehand with an axial duct in which the mandrel can pass, but generally it is preferable to perforate the billet once it has been placed in its receptacle.
Consequently, the conventional sequence of operations will be as follows:
When the billet has been placed inside the receptacle of the container closed at one end by the die-holder insert, the billet is first rammed within its receptacle by pushing it against the die by means of the movable rammer in direct drawing or by means of a special stop in reverse drawing. The purpose of this operation is to apply the billet against the walls of the receptacle and make it adhere to them.
Perforation is then carried out by advancing the mandrel which passes axially through the billet. If the advance of the mandrel is stopped before it has emerged in the die on the other side of the billet, the axial orifice remains closed by a metal partition of small thickness which remains at the end of the tube which is formed by drawing and in which a vacuum is therefore generated. The atmospheric pressure acting on the outside of the drawn tube tends to crush and deform the latter which can be flattened completely if it is of small thickness. Consequently, it is generally preferable to perforate the billet completely before drawing, by advancing the mandrel during perforation until it penetrates within the die. The result of this is that the end of the mandrel causes a small piece of metal, called a plug, corresponding to the partition which previously closed the end of the tube, to drop into the die. The presence of this plug is particularly annoying in reverse drawing, since it then falls inside the tubular rammer and is difficult to remove.